An improved understanding of the brain is considered a key to the continued advancement of modern medical technology and, therefore, human health and well-being. But, its scale (100 billion neurons) and density of connections (1 neuron connecting to many thousands) present experimental challenges for designing neural interfaces. Getting information into and out of its neural networks requires an ability to access the activity of many thousands of neurons simultaneously, which requires a commensurately high number of individual recording channels and electrodes. The technology for practical electrical recording in the brain has not significantly advanced in decades, however, and the limited number of channels (≤1000) in state of the art neural probes is a limiting factor for understanding brain function.
A better understanding of brain functionality would also enable improved treatment of many neurological diseases, such as Parkinson's disease. Deep-brain stimulation (DBS) is currently used to stimulate specific parts of the brain (e.g., basal ganglia nuclei, such as the subthalamic nucleus or internal part of the globus pallidus, etc.) with electrical impulses via a surgically implanted probe containing one or more electrodes. Unfortunately, the effectiveness of conventional DBS methods is limited by the available probe technology. Currently available therapeutic DBS probes normally include only one or a few electrodes. While such a probe enables control of stimulation frequency and amplitude, it provides little or no control over the spatial pattern of the excitation signals—in contrast to the present understanding of the manner in which neural circuits process information. In addition, single-electrode probes cannot provide the nuanced intervention desirable for the treatment of complex neuropsychiatric disorders, such as individually activating corticofugal axons with highly localized stimulation.
As a result, to understand and treat diseases such as Parkinson's, depression, and post-traumatic stress disorder, and the like, a more sophisticated stimulation method that can deliver patterned excitation over a fairly large region to deep brain areas is needed—ideally with single-cell-level resolution. Multi-electrode probes are seen as a means for providing such capability. To that end, significant effort is being directed toward the development of high-density-electrode arrays, such as comb-like arrangements of silicon microelectrodes and flat lithography-based nano-electrode arrays.
Silicon microelectrodes are sculpted from conventional silicon wafers, which are normally used for planar fabrication of integrated circuits. A typical silicon microelectrode includes a dense array of electrodes arranged along the length of each “tooth” of the comb. These probes are capable of dense neural sampling along the longitudinal axis of each tooth; however, they are bulky. In addition, due to fabrication-based limitations on the aspect-ratio with which the probe elements can be made, it is difficult, if not impossible, to make a silicon comb with teeth that are both long enough and fine enough to densely sample neurons within a useful volume of brain tissue. Still further, these arrays are typically limited to approximately one hundred channels, which enables access to only about 0.5% of the cells in a single cortical circuit.
Lithographically fabricated nano-electrode arrays can have submicron pitch, which affords good lateral sampling density. In similar fashion to silicon electrodes, however, the aspect ratio limitations for such nanowires prove limiting. Growing oriented nanowires of >100 micrometers in length is extremely difficult. As a result, without invasive surgery, such shallow electrode structures enable recording from only those neurons that are located on, or very near, the surface of the brain.
A practical, high-density, massively parallel, single-cell-resolution probe that can access to deep-brain activity would be revolutionary for neurophysiology. It would enable an improved fundamental understanding of neural-network behavior, as well as improved deep-brain stimulation treatments. Such a probe, however, remains unrealized in the current state of the art.